FOREIGN OBJECTS IN BIRD NESTS

Transcription

1 FOREIGN OBJECTS IN BIRD NESTS MICHAEL R. CONOVER Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station, Box 1106, New Haven, Connecticut USA ABSTRACT.--Up to 10% of the Ring-billed Gull (Larus delawarensis) and 6% of the California Gull (L. californicus) nests in some Washington colonies contained pebbles or other round foreign objects. These foreign objects, or pseudo-eggs, were more similar in size and shape to gull eggs than were randomly selected pebbles. Nests containing pseudo-eggs were most common in colonies where pebbles littered the ground. Pseudo-eggs were also more common in 1- and 2-egg clutches than in 3-egg clutches. In many respects, the gulls treated the pseudo-eggs as they did their own eggs. If pseudo-eggs were placed outside a nest, the adults rolled them back. If all eggs were removed from the nest, Ring-billed continued to incubate nests containing pseudo-eggs but not empty nests. These results provide more support for the hypothesis that these objects occur in nests because birds mistake them for eggs than for the hypothesis that these objects benefit the birds by acting as incubation stimuli. Received 2 October 1984, accepted 5 April FOREIGN object such as rocks and exotic eggs occasionally have been found inside the nests of incubating birds, particularly among groundnesting species. Knight and Erickson (1977) found stones and pine cones inside the nests of Canada Geese (Branta canadensis) nesting in Washington state. Coulter (1980) found stones in the nests of Western (Larus occidentalis) in the Farallon Islands in California and also in the nests of Common Terns (Sterna hirunda) on Great Gull Island in New York. Sugden (1947) found stones in the nests of American Avocets (Recurvirostra americana) in Utah. He also discovered eggs, apparently from other species, in some California Gull (Larus californicus) nests. Henceforth, I will refer to these foreign objects found inside nests as pseudo-eggs. A pseudo-egg is defined as any object greater than 2 cm in both width and length that sits entirely within the nest cup. Objects that are incorporated into the nest cup or protrude through it are not considered pseudo-eggs. I used the relatively high frequency of pseu- do-eggs in the nests of Ring-billed (L. delawarensis) and California gulls in several Washington and Oregon colonies to examine this phenomenon and to answer the following questions. (1) What are the physical characteristics of pseudo-eggs, and how do they differ from randomly selected objects? (2) Do pseudoegg frequencies differ among colonies or between Ring-billed and California gulls nesting in the same colony? (3) Where do the pseudo- eggs come from? Do they originate near the nest, or are they brought in from greater distances? (4) Are pseudo-eggs found with equal frequency in nests with different-size clutches? And (5) Do incubating gulls treat pseudo-eggs like their own eggs? That is, will they roll back pseudo-eggs if they are placed outside the nest? Will gulls continue to incubate nests containing only pseudo-eggs? The answers to these questions, in turn, shed light on one of the most intriguing questions about this phenomenon: Why do birds not only tolerate these objects in their nests but bring them there in the first place? METHODS The experiments were conducted in 1980 and 1981 in five Ring-billed and California gull colonies located in Washington and Oregon: Island 18, Island 20, Little Memaloosa, Potholes Reservoir, and Threemile Canyon colonies. These colonies are described elsewhere (Conover et al. 1979). All observations and experiments were conducted in May when the gulls were at least halfway through the incubation period. Pseudo-egg characteristics.--fifty pseudo-eggs were randomly collected from Ring-billed Gull nests in the Island 20 colony, as were 50 pebbles from the ground surface in the same part of the colony. This was done by establishing a numbered grid system in the colony or subcolony that was to be surveyed and selecting 50 grid points from a table of random numbers. The nearest pseudo-egg to that point and the nearest pebble lying on the surface were then collected. All collected objects were weighed and measured. To assess whether the gulls were randomly 696 The Auk 102: October 1985

2 October 1985] Foreign Objects in Nests 697 Fig. 1. Ring-billed Gull eggs (in box) and randomly collected pseudo-eggs. selecting objects for pseudo-eggs, I used a Student's t-test to determine whether the sizes or weights of pseudo-eggs were significantly different from those of the pebbles outside the nest. Pseudo-egg frequencies.--i examined the interspecific or intraspecific differences in pseudo-egg frequencies. The question of where the objects for pseudoeggs originated was addressed by comparing the frequency of nests with pseudo-eggs to the type of subjects were numerous. I compared the frequency of nests containing pseudo-eggs in the pebble-strewn areas to nests in other areas where pebbles were lacking using a Chi-square contingency table corrected for continuity. Retrieval of pseudo-eggs.--in this experiment I examined whether the gulls individually recognized their pseudo-eggs and would roll them back into their nests when the pseudo-eggs were placed outside. I strate found near the nests. If objects for pseudo-eggs removed pseudo-eggs from several Ring-billed and come from near the nest, then nests containing pseudo-eggs should be rare in areas devoid of suitable objects. For example, the gulls at the Potholes Res- California gull nests at the Island 20 and Three-mile Canyon colonies and placed them 5-15 cm outside each nest. I also took eggs from other nests and placed ervoir colony nested on sand dunes, and rocks or them the same distance outside. All nests were other suitable objects for pseudo-eggs were rare inside the colony. Suitable objects for pseudo-eggs were also rare at the Little Memaloosa colony, which was on solid rock. In contrast, parts of the colonies on Island 18, Island 20, and Three-mile Canyon were situated in a pebble-strewn area where suitable obchecked after h to see how many eggs and pseudo-eggs had been rolled back into the nests. Retrieval rates for eggs and pseudo-eggs were compared using a Chi-square contingency table corrected for continuity. Relationship between pseudo-egg frequencies and clutch

3 698 MicIa^E. R. CONOVER [Auk, Vol. 102 T^BLE 1. Weight and size of Ring-billed Gull eggs, pseudo-eggs (n = 59), and randomly selected pebbles from outside the nest (n = 64). Ring- Randomly billed selected Gull Pseudo-eggs pebbles eggs (œ + 95% (œ + 95% (œ) CI a) CI-) t Weight Length Width Depth ' CI = confidence interval. Incubation of pseudo-eggs.--in this experiment I examined whether gulls would continue to incubate their nests after all of their eggs had been removed and only their pseudo-eggs remained. Eggs were removed from 23 Ring-billed Gull nests at Island 20; 11 contained pseudo-eggs and the rest did not. The nests were checked later to determine whether they were still being incubated. An excellent opportunity occurred on the fourth day, when it rained for several hours; a dry nest was a clear indication that it had been incubated almost continuously for the last several hours. 20 colonies, the frequency of nests with pseudo-eggs did not vary significantly between 1980 and 1981 for either Ring-billed (X 2 = 0.45, df = 2 in this and in all other tests unless oth- erwise indicated, P > 0.05) or California (X 2 = 1.36). Consequently, data from both years were combined for the rest of this study. In California, 1.0% of the examined nests (n = 2,503) contained pseudo-eggs, as did 3.8% of the Ring-billed Gull nests (n = 2,182). The difference between the two species was statistically significant (X 2 = 13.89, P < 0.01). There were also intraspecific differences in pseudo-egg frequencies. The frequency of nests size.--to determine whether pseudo-eggs occurred containing pseudo-eggs varied significantly with equal frequency in nests with different-size among Ring-billed Gull colonies (X 2 = 45.98, clutches, I examined nests in randomly located plots df = 3, P < 0.01) and California Gull colonies in pebble-strewn areas of the Island 18, Island 20, (X 2 = 78.68, df = 4, P < 0.01). Pseudo-eggs were and Three-mile Canyon colonies and recorded the very common in all of the colonies with a pebpercentage of 1-, 2-, and 3-egg nests that contained pseudo-eggs. Chi-square tests were then conducted ble substrate (Table 2); 6.7% of the Ring-billed Gull nests and 1.6% of the California Gull nests to determine whether the frequency of nests containing pseudo-eggs varied significantly among nests in these areas contained them. In contrast, in with different clutch sizes. areas with a sandy or solid rock surface only 2 pseudo-eggs were found among the 967 Ringbilled Gull nests examined and none among the 951 California Gull nests. These differences in pseudo-egg frequency between pebble and nonpebble areas were statistically significant for both Ring-billed (X 2 = 60.50, P < 0.01) and California gulls (X 2= 13.89, P < 0.01). These re- suits indicate that the presence of pebbles near a nest increases the probability of that nest containing pseudo-eggs, presumably because most pseudo-eggs originate from near the nest site. Retrieval of pseudo-eggs. --Forty percent of the RESULTS Ring-billed rolled the pseudo-eggs back into their nests less than 12 h after I placed the Pseudo-egg charactehstics.--all foreign objects pseudo-eggs outside; 67% rolled back their own found in gull nests were round, smooth objects eggs (Table 3). This difference was not statisti- (usually pebbles; Fig. 1). Pseudo-eggs from cally significant (X 2 = 3.34). In California, Ring-billed Gull nests differed significantly 45% rolled back pseudo-eggs and 51% rolled from pebbles randomly selected from inside the back their own eggs; this difference also was colony in length, width, and depth but not in not statistically significant (X 2 = 0.00). weight (Table 1). Pseudo-eggs were less vari- Relationship between pseudo-egg frequencies and able in size and shape and were closer to the clutch size.--the frequency of pseudo-eggs varsize and shape of an actual Ring-billed Gull ied significantly among different-size clutches egg than were randomly selected pebbles (Ta- in both Ring-billed (X 2 = 37.02, P < 0.01) and ble 1). In fact, the mean length and width of California gulls (X 2 = 21.57, P < 0.01). The fre- Ring-billed Gull eggs (based on egg-dimension quency of pseudo-eggs was higher in nests data from Vermeer 1970) lie within the 95% containing 1 or 2 eggs than in those containing confidence interval of the mean length and 3 eggs (Table 4). width of pseudo-eggs but not of randomly se- Incubation of pseudo-eggs.--four days after all lected pebbles. eggs were removed from 11 Ring-billed Gull Pseudo-egg frequencies.--athe Island 18 and nests so that only pseudo-eggs remained, 8 of

4 October 1985] Foreign Objects in Nests 699 T^I LE 2. Frequency of nests containing pseudo-eggs in different colonies. California Gull nests Ring-billed Gull nests No. % with No. % with Colony Substrate examined pseudo-eggs examined pseudo-eggs Little Memaloosa Solid rock % Three-mile Canyon Pebble % % Island 18 Pebble % % Island 20 Pebble % % Sand % % Potholes Reservoir Sand % % the nests (73%) were still being incubated. In contrast, after all eggs were removed from 12 nests not containing pseudo-eggs, incubation of all of these nests ceased within the 4-day observation period. This difference was statistically significant (X 2 = 10.37, P < 0.01). My finding that pseudo-eggs were more common in 1- and 2-egg clutches than in 3-egg clutches supports this hypothesis. The hypothesis, however, does not explain some of my other observations. For example, if this hypothesis is correct, why do gulls continue to incubate nests after all eggs have been re- DISCUSSION moved and nothing remains but pseudo-eggs? Also, I found that pseudo-egg frequencies were One of the most intriguing questions of the much lower in colonies or subcolonies that pseudo-egg phenomenon is why birds not only tolerate the presence of foreign objects in their lacked a pebble substrate. If pseudo-eggs provide a benefit to the birds, then gulls lacking nests but actively bring them into their nests. suitable objects near their nests should bring One hypothesis is that the gulls recognize them from farther away. pseudo-eggs as foreign objects but keep them in the nests because they benefit the birds. The second general hypothesis is that bringing and keeping pseudo-eggs in the nest is For example, Coulter (1980) observed that gulls maladaptive and occurs because gulls mistake incubating three eggs had longer incubation the pseudo-eggs for something else, such as for bouts, fewer resettlings, and greater hatching food (mistaken-food hypothesis) or their own success than those incubating smaller clutches. eggs (mistaken-egg hypothesis). The mistaken- He speculated that foreign objects are added to the nest by incubating gulls and that these objects are an important incubation stimulus in 1- or 2-egg clutches. food hypothesis is an extrapolation of Sugden's (1947) and Twomey's (1948) hypothesis for the presence of exotic eggs in gulls' nests. They noted that California regularly eat eggs, and Twomey once observed a California Gull T^I LE 3. Frequency with which eggs and pseudoeggs were rolled back into nests within 12 h after being placed outside. California Clutch size Total No. eggs removed % rolled back No. pseudo-eggs removed % rolled back Ring-billed No. eggs removed % rolled back No. pseudo-eggs removed % rolled back TABLE 4. Frequency of pseudo-eggs in nests containing different-size clutches. Ring-billed 1-egg clutches No. examined 95 % with pseudo-eggs egg clutches No. examined 356 % with pseudo-eggs egg clutches No. examined 1,030 % with pseudo-eggs 3.6 California

5 700 MICHAEL R. CONOVER [Auk, Vol. 102 regurgitate a duck egg near its nest. They spec- inside their nests (Miller and Conover 1983). ulated that if a regurgitated egg landed inside Nevertheless, the adoption of pebbles seems the nest cup, the gull might incubate it along more extraordinary. Although the pseudo-eggs with its own eggs. According to this hypothe- are approximately the same size, shape, and sis, a gull must mistake a pebble for an egg, color as real eggs, there are still substantial difswallow it, and regurgitate near the nest cup. ferences (Fig. 1). Adoption of pebbles may oc- It seems unlikely, however, that gulls regularly cur, however, because little is lost in reproducmistake pebbles for food. In addition, gulls do tive fitness if a gull mistakenly brings a pebble not start to regurgitate food around the nest into its nest. But if a gull makes the opposite until after the chicks hatch (Miller and Con- mistake and leaves an egg outside the nest owover 1983), but it is during the incubation pe- ing to a failure to recognize it, the gull's loss riod that pseudo-eggs occur. It is unlikely that in reproductive fitness is substantial. Hence, if these events would occur often enough to ac- a gull is in doubt about the identity of an object count for the pseudo-egg frequencies found in outside the nest, its reproductive fitness may this study (up to 10% in some Ring-billed Gull be increased by bringing the object into the colonies). Finally, gulls generally forage away nest. from the colony, so most pseudo-eggshould originate from outside the colony; I found, ACKNOWLEDGMENTS however, that pseudo-eggs came from near the nest site. I thank D. E. Aylor, D. O. Conover, M. C. Coulter, B. W. Kent, and K. Vermeer for their comments on an earlier draft of this manuscript. Part of this project The other possibility is that pseudo-eggs occur because the birds cannot clearly discriminate between them and their own eggs (mistaken-egg hypothesis). Hence, these objects are rolled into the nest and incubated because the was conducted while the author was on an NSF National Needs Postdoctoral Fellowship. bird mistakes them for its own eggs. The LITERATURE CITED strongest support for the mistaken-egg hypoth- BUCKLEY, P. A., & F. G. BUCKLEY Individual esis comes from the finding that gulls treat egg and chick recognition by adult Royal Terns pseudo-eggs like their own eggs. If pseudo-eggs (Sterna maxima maxima). Anim. Behav. 20: 457- are placed outside the nest, gulls roll them back 462. in; if all eggs are removed, gulls continue to incubate nests containing only pseudo-eggs. It is unclear why gulls would do these things unless they mistook the pseudo-eggs for real eggs. CONOVER, M. R., B.C. THOMPSON, R. E. FITZNER, & D. E. MILLER Increasing populations of Ringbilled and California gulls in Washington state. Western Birds 10: Also, the mistaken-egg hypothesis would pre- COULTER, M.C Stones: an important stimulus dict that objects for pseudo-eggs come from near for gulls and terns. Auk 97: HANSON, W. C., & L. L. EBERHARDT A Columthe nest site, as was found in this study. The bia River Canada Goose population mistaken-egg hypothesis also could account for Wildl. Monogr. 28: Hanson and Eberhardt's (1971) observation of KNIGHT, R. L., & A. W. ERICKSON Objects intwo Canada Goose nests that were incubated corporated within clutches of the Canada Goose. while containing rocks but no eggs. The mis- Western Birds 8: 108. taken-egg hypothesis, however, does not explain why pseudo-egg should be more common in 1- and 2-egg nests than in 3-egg nests. If the mistaken-egg hypothesis is correct, MILLE, D. E., & M. R. CONOVER Chick vocal patterns and non-vocal stimulation as factors instigating parental feeding behaviour in the Ringbilled Gull. Anim. Behav. 31: these gulls exhibit a surprising inability to dis- SUGDEN, J. W Exotic eggs in nests of California. Condor 49: tinguish between their own eggs and other ob- TWOMEY, A.C Californiaandexoticeggs. jects. In contrast, some terns can discriminate Condor 50: between their own eggs and those of other VERMEER, K Breeding biology of California conspecifics (Buckley and Buckley 1972). Ring- and Ring-billed gulls: a study of ecological adbilled, however, do not have this ability aptation to the inland habitat. Can. Wildl. Serv. and will readily accept conspecific eggs placed Rept. Set. 12: 1-51.